1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor optical integrated device, a method for fabricating a semiconductor laser device, a semiconductor optical integrated device, and a semiconductor laser device.
2. Description of the Related Art
In a semiconductor optical integrated device, a light emitting element (e.g., a semiconductor laser and a light emitting diode), an optical waveguide element (including an optical divider, an optical coupler, a filter, a modulator, and the like), and a light receiving element (e.g., a photodiode) are integrated on one semiconductor substrate. Such a semiconductor optical integrated device is important for realizing a small, inexpensive, highly functional optical device.
A method for forming the above elements monolithically on a semiconductor substrate by one crystal growth step is known. This method is advantageous over a fabrication method in which the respective elements are arranged by alignment, in that since an alignment process is not necessary, production yield improves, the cost can be reduced, and the size of the entire device can be easily reduced.
In a semiconductor optical integrated device, in order to increase the optical coupling efficiency among a light emitting element, an optical waveguide element, and a light receiving element, core layers of the respective elements are preferably positionally aligned to continue in series. Moreover, in order to avoid light emitted from the light emitting element being subjected to excessive absorption loss when the light propagates in the optical waveguide element, the optical waveguide element must be transparent to light emitted from the light emitting element. For this purpose, the forbidden bandwidth of the core layer of the optical waveguide element needs to be wider than that of the core layer (active layer) of the light emitting element.
As a method for fabricating a semiconductor optical integrated device which satisfies the above requirements, partly disordering a core layer composed of a quantum well structure is proposed. For example, Japanese Laid-Open Publication No. 3-89579 discloses a method for integrating a semiconductor laser element having a multiple quantum well (MQW) as an active layer (a distributed feedback (DFB) laser) and an optical waveguide element (an optical modulator) having a disordered MQW as a core layer, as well as a device obtained by this integration method. Hereinbelow, the disclosed method, as well as the construction of the device, will be described with reference to FIGS. 15A to 15C.
First, referring to FIG. 15A, a p-type InGaAsP optical waveguide layer 19, a MQW core layer 14, an n-type InP upper cladding layer 15, and an n-type InGaAsP contact layer 16 are sequentially formed by metal organic vapor phase epitaxy (MOVPE) on a p-type InP substrate 11 on which a diffraction grating 20 is partly formed. The MQW core layer 14 is composed of ten periods of an InGaAs well layer (thickness: 8 nm) and an InGaAsP barrier layer (corresponding to xcexg=1.3 xcexcm, thickness: 11 nm).
Referring to FIG. 15B, a region M of the resultant structure which is to be a DFB laser is covered with a dielectric film 41 such as an SiO2 film, while in a region N which is to be an optical modulator, sulfur is diffused so as to be distributed from the surface of the contact layer 16 to the core layer 14. The MQW structure of the core layer 14 is destroyed due to the sulfur diffusion, turning the core layer 14 into a disordered layer 42.
Thereafter, referring to FIG. 15C, a buried structure for controlling the transverse mode is formed in the following manner. First, the resultant structure is etched to form a mesa along the center in the length direction of the structure. Fe-doped high-resistance InP buried layers 43 are grown by MOVPE on both sides of the mesa along the length of the structure. Electrodes 17 and 18 are formed on both surfaces of the resultant structure, and then a separation groove 25 is formed by etching to separate the two regions. The resultant wafer is cleaved to obtain the semiconductor optical integrated device as shown in FIG. 15C.
The above conventional fabrication method and construction of the optical integrated device have a feature that the DFB laser region M includes an active layer composed of an MQW structure and the optical modulator region N includes a semiconductor layer in which the same MQW structure is disordered (disordered layer). When a particular impurity is introduced into an MQW structure by diffusion, ion implantation, or the like, the MQW structure is disordered, changing into a bulk semiconductor layer having an average composition of the MQW structure, and thus slightly increasing the forbidden bandwidth. In other words, the optical modulator region N becomes transparent to light emitted from the DFB laser. The optical modulator which is transparent to the laser light allows the light to propagate therein without inducing light loss. Thus, when an electric field is applied to the electrodes of the optical modulator, the laser light propagating in the optical modulator can be modulated with high efficiency.
In the above conventional method for fabricating a semiconductor optical integrated device, the light emitting element, the light receiving element, and the optical waveguide element can be fabricated simultaneously without the necessity of a complicated processing step. In this conventional method, the core layers of the respective elements, i.e., the light emitting layer of the light emitting element, the light absorption layer of the light receiving element, and the optical guide layer of the optical waveguide element, are simultaneously formed by crystal growth as one continuous layer. Accordingly, the semiconductor optical integrated device fabricated by this method has no displacement or seam between the layers and thus provides a large optical coupling efficiency.
The technique of partly disordering a quantum well core layer used in the above conventional method is also utilized in methods for fabricating other types of semiconductor lasers. For example, Y. Suzuki et al., Electronics Letters, Vol.20 (1984) pp.383-384 describes that high output power can be realized for a semiconductor laser having an MQW active layer by introducing impurities only into end faces of the semiconductor laser to disorder the end faces, forming xe2x80x9cwindow-stripesxe2x80x9d which do not absorb light.
Japanese Laid-Open Publication No. 7-106697 discloses a gain-coupled distributed feedback semiconductor laser having an absorptive diffraction grating for periodically changing the absorption amount, which is obtained by periodically disordering an absorptive MQW guide layer disposed immediately above an active layer.
Japanese Laid-Open Publication No. 3-49285 discloses a gain-coupled distributed feedback semiconductor laser having a gain diffraction grating for periodically changing the gain of an active layer, which is obtained by periodically disordering the MQW active layer.
In the conventional fabrication method described with reference to FIGS. 15A to 15C, impurities are introduced at a high concentration by a technique such as heat diffusion or combination of ion implantation and annealing.
In such heat diffusion and annealing, a resultant substrate after the completion of crystal growth needs to be kept at a high temperature for an extended period of time. For example, J. J. Coleman et al, Appl. Phys. Lett., Vol.40 (1982) p.904 describes a disordering method by Si ion implantation and annealing at a temperature as high as 675xc2x0 C. for a period of time as long as four hours. When an p-n junction device such as a semiconductor laser is subjected to such a high-temperature, long-time heat treatment, impurities which should not be diffused, such as impurities doped in a cladding layer during crystal growth, are also diffused. This results in autodoping of impurities to the active layer, thereby greatly reducing the light emitting efficiency. As a result, significant degradation in the characteristics of the semiconductor laser, such as marked increase in operating power and reduction in lifetime, is exhibited.
In view of the foregoing, an object of the present invention is to provide a method for fabricating a semiconductor optical integrated device in which a light emitting element, a light receiving element, and an optical waveguide element are integrated without the necessity of high-temperature heat treatment, as well as the semiconductor optical integrated device fabricated by such a method. In particular, an object of the present invention is to provide a method for fabricating a semiconductor optical integrated device in which a) a high-temperature long-time heat treatment process is not required, b) a light emitting element, a light receiving element, and an optical waveguide element can be simultaneously formed by one crystal growth step without the necessity of complicated processing, and c) a light emitting layer of the light emitting element, a light absorption layer of the light receiving element, and a core layer of the optical waveguide element continue as one layer without any displacement or seam between the layers thus reducing excessive light loss.
Another object of the present invention is to provide a semiconductor optical integrated device in which a light emitting element, a light receiving element, and an optical waveguide element are formed simultaneously.
The method for fabricating a semiconductor device of this invention includes the step of: forming a first compound semiconductor layer by crystal growth on a surface of a semiconductor substrate which includes a plurality of crystal planes having different orientations exposed due to a concave portion and/or a convex portion formed on the semiconductor substrate, the first compound semiconductor layer containing nitrogen and a V group element other than nitrogen.
In one embodiment of the invention, the step of forming a first compound semiconductor layer includes the step of supplying a material of nitrogen and a material of the V group element other than nitrogen alternately.
In another embodiment of the invention, the step of forming a first compound semiconductor layer includes the step of controlling the composition ratio of nitrogen to the V group element other than nitrogen depending on the difference in the orientation of the crystal planes.
In still another embodiment of the invention, the first compound semiconductor layer comprises a well layer of a quantum well structure.
In still another embodiment of the invention, the V group element other than nitrogen is at least one type selected from the group consisting of arsenic, phosphorus, and antimony.
According to another aspect of the invention, a method for fabricating a semiconductor optical integrated device is provided. The device includes the step of: forming a core layer and an upper cladding layer composed of at least one layer sequentially by crystal growth on a surface of a semiconductor substrate which includes a plurality of crystal planes having different orientations exposed due to a concave portion and/or a convex portion formed on the semiconductor substrate, the core layer including at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen.
In one embodiment of the invention, the plurality of crystal planes are selected from planes tilted from (100) plane in an A direction (a [011] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and an optical waveguide element is formed on a plane having a smaller tilt angle while a light emitting element and/or a light receiving element are formed on a plane having a larger tilt angle.
In another embodiment of the invention, the plurality of crystal planes are selected from planes tilted from (100) plane in a B direction (a [0-11] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and an optical waveguide element is formed on a plane having a larger tilt angle while a light emitting element and/or a light receiving element are formed on a plane having a smaller tilt angle.
Alternatively, the method for fabricating a semiconductor optical integrated device of this invention includes the steps of: forming at least one set of continuous grooves or mesas on a semiconductor substrate having {100} plane as a principal plane, side walls of the continuous grooves or mesas being made of a plane tilted from (100) plane in an A direction (a [011] direction) by an angle in a range of 0 to 55xc2x0 or a plane crystallographically equivalent to such a plane, the continuous grooves or mesas having substantially the same depth and different widths; forming a waveguide structure by sequentially forming by crystal growth a lower cladding layer composed of at least one layer, a core layer, and an upper cladding layer composed of at least one layer, the core layer including at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen; forming a light emitting element and/or a light receiving element using a portion of the waveguide structure located above the groove or mesa having a smaller width; and forming an optical waveguide element using a portion of the waveguide structure located above the groove or mesa having a larger width.
Alternatively, the method for fabricating a semiconductor optical integrated device of this invention includes the steps of: forming at least one set of continuous grooves or mesas on a semiconductor substrate having {100} plane as a principal plane, side walls of the continuous grooves or mesas being made of a plane tilted from (100) plane in a B direction (a [0-11] direction) by an angle in a range of 0 to 55xc2x0 or a plane crystallographically equivalent to such a plane, the continuous grooves or mesas having substantially the same depth and different widths; forming a waveguide structure by sequentially forming by crystal growth a lower cladding layer composed of at least one layer, a core layer, and an upper cladding layer composed of at least one layer, the core layer including at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen; forming a light emitting element and/or a light receiving element using a portion of the waveguide structure located above the groove or mesa having a larger width; and forming an optical waveguide element using a portion of the waveguide structure located above the groove or mesa having a smaller width.
According to still another aspect of the invention, a method for fabricating a semiconductor laser device is provided. The method includes the step of: forming a lower cladding layer composed of at least one layer, an active layer, and an upper cladding layer composed of at least one layer sequentially on a semiconductor substrate by crystal growth, the active layer including at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen, the substrate having a convex portion and/or a concave portion to expose a plurality of crystal planes having orientations different between end face portions and a center portion in a resonator direction.
In one embodiment of the invention, the plurality of crystal planes are at least selected from planes tilted from (100) plane in an A direction (a [011] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and the crystal plane in the end face portions has a smaller tilt angle while the crystal plane in the center portion has a larger tilt angle.
In another embodiment of the invention, the plurality of different crystal planes are at least selected from planes tilted from (100) plane in a B direction (a [0-11] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and the crystal plane in the end face portions has a larger tilt angle while the crystal plane in the center portion has a smaller tilt angle.
Alternatively, the method for fabricating a semiconductor laser device of this invention includes the step of: producing an absorptive diffraction grating by forming a diffraction grating made of periodic concave and convex portions in the vicinity of an active layer and forming at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen above the diffraction grating so that the concave and convex portions of the diffraction grating are traced.
Alternatively, the method for fabricating a semiconductor laser device of this invention includes the step of: producing a gain diffraction grating by forming a diffraction grating made of periodic concave and convex portions and forming at least one first compound semiconductor layer containing nitrogen and a V group element other than nitrogen above the diffraction grating so that the concave and convex portions of the diffraction grating are traced.
Alternatively, the method for fabricating a semiconductor optical integrated device of this invention includes the step of: fabricating a semiconductor laser device by the method according to the present invention as a semiconductor laser element, simultaneously with other elements constituting the semiconductor optical integrated device.
According to still another aspect of the invention, a semiconductor optical integrated device is provided. The device is a semiconductor device fabricated by the method according to the present invention, including at least a light emitting element and/or a light receiving element and an optical waveguide element which are integrated on one substrate, wherein both a core layer of the light emitting element and/or the light receiving element and a core layer of the optical waveguide element include the first compound semiconductor layer, and a nitrogen mole fraction of the first compound semiconductor layer is larger in the light emitting element and/or the light receiving element than in the optical wavelength element.
In one embodiment of the invention, the light emitting element and/or the light receiving element and the optical waveguide element are formed on a substrate having planes selected from planes tilted from (100) plane in an A direction (a [011] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, the light emitting element and/or the light receiving element being formed on one of the planes having a larger tilt angle and the optical waveguide element being formed on another of the planes having a smaller tilt angle.
In another embodiment of the invention, the light emitting element and/or the light receiving element and the optical waveguide element are formed on a substrate having planes selected from planes tilted from (100) plane in a B direction (a [0-11] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, the light emitting element and/or the light receiving element being formed on one of the planes having a smaller tilt angle and the optical waveguide being formed on another of the planes having a larger tilt angle.
According to still another aspect of the invention, a semiconductor laser device is provided. The device is a semiconductor device fabricated by the method according to the present invention, wherein an active layer of the semiconductor laser device includes the first compound semiconductor layer, and a nitrogen mole fraction of the first compound semiconductor layer is larger in an end face portion of a resonator than in the center portion of the resonator in the resonator direction.
In one embodiment of the invention, the semiconductor laser device is formed on a substrate having planes selected from planes tilted from (100) plane in an A direction (a [011] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and the center portion of the first compound semiconductor layer is formed on one of the planes having a larger tilt angle while the end face portion of the first compound semiconductor layer is formed on another of the planes having a smaller tilt angle.
In another aspect of the invention, the semiconductor laser device is formed on a surface of a substrate having planes selected from planes tilted from (100) plane in a B direction (a [0-11] direction) by an angle in a range of 0 to 55xc2x0 and planes crystallographically equivalent to such planes, and the center portion of the first compound semiconductor layer is formed on one of the planes having a smaller tilt angle while the end face portion of the first compound semiconductor layer is formed on another of the planes having a larger tilt angle.
Alternatively, the semiconductor laser device of the present invention is a semiconductor device fabricated by the method according to the present invention, wherein the first compound semiconductor layer is formed on a diffraction grating made of periodic concave and convex portions formed in the vicinity of an active layer, the first compound semiconductor layer tracing the concave and convex portions, a nitrogen mole fraction of the first compound semiconductor layer periodically changes in correspondence with the periodic concave and convex portions of the diffraction grating, and portions of the first compound semiconductor layer having a larger nitrogen mole fraction absorb light from the active layer, to establish an absorptive diffraction grating which generates periodic modulation of gain.
Alternatively, the semiconductor laser device is a semiconductor device fabricated by the method according to the present invention, wherein the first compound semiconductor layer is formed on a diffraction grating made of periodic concave and convex portions, the first compound semiconductor layer tracing the concave and convex portions, and a nitrogen mole fraction of the first compound semiconductor layer periodically changes in correspondence with the periodic concave and convex portions of the diffraction grating, to generate periodic modulation of a gain of the active layer.
Hereinbelow, the function of the present invention will be described.
The fabrication method of the present invention utilizes the principle that the nitrogen mole fraction of the first compound semiconductor layer slightly changes depending on the base crystal plane and this slight change of the nitrogen mole fraction largely changes the forbidden bandwidth. Utilizing this principle, it is possible to form portions having different forbidden bandwidths by one crystal growth step without the necessity of heat treatment. This function can be obtained by controlling the nitrogen mole fraction based on the base crystal plane.
The change in the nitrogen mole fraction depending on the base crystal plane is intensified by supplying V group materials alternately. This allows for obtaining the above function more effectively.
The first compound semiconductor layer constitutes the well layer of the quantum well structure. A distribution is generated in a lattice constant in correspondence with the distribution of the N mole fraction. However, the well layer is so thin that it can have a distortion inside and thus no lattice defect is generated due to the lattice constant distribution.
A large change in the forbidden bandwidth is obtained by a slight change in the nitrogen mole fraction by selecting at least one type of element from the group consisting of arsenic, phosphorus, and antimony, as an element to be mixed together with nitrogen. This allows for obtaining a crystal suitable for realizing the device according to the present invention.
By utilizing the principle that the nitrogen mole fraction of the first compound semiconductor layer slightly changes depending on the base crystal plane and this slight change of the nitrogen mole fraction largely changes the forbidden bandwidth, it is possible to form a plurality of waveguide structures having different forbidden bandwidths in the core layer by one crystal growth step without the necessity of heat treatment.
A semiconductor laser device having window structures at end faces can be easily obtained by forming by crystal growth a compound semiconductor layer containing nitrogen and a V group element other than nitrogen on a substrate having a plurality of planes with different orientations exposed thereon.
A semiconductor laser device having an absorptive periodic structure and a semiconductor laser device having a gain periodic structure can be easily obtained by forming by crystal growth a compound semiconductor layer containing nitrogen and a V group element other than nitrogen as components on a substrate having a plurality of planes with different orientations exposed thereon.
According to the present invention, the structure having a forbidden bandwidth distribution can be formed by one crystal growth step without the necessity of heat treatment. This allows for fabricating devices having excellent characteristics by a reduced number of fabrication steps.
The forbidden bandwidth distribution can be intensified by forming portions having slightly different nitrogen mole fractions. According to the present invention, the compound semiconductor layer containing nitrogen and a V group element other than nitrogen is used as the core layer of the semiconductor optical integrated device, and the forbidden bandwidth of the light emitting element and/or the light receiving element is made larger than that of the optical waveguide element. By this setting, therefore, light emitted from the light emitting element can propagate through the optical waveguide element with low light loss and can be absorbed by the light receiving element with good efficiency.
According to the present invention, a large forbidden bandwidth distribution can be obtained by slightly changing the nitrogen mole fraction. Therefore, it is suitable for the semiconductor laser device to form the compound semiconductor layer containing nitrogen and a V group element other than nitrogen so that the nitrogen mole fraction is slightly different between the portions thereof at the end faces and the portion thereof in the center.
According to the present invention, a large forbidden bandwidth distribution can be obtained by slightly changing the nitrogen mole fraction. Therefore, it is suitable for the semiconductor laser device to use the portions having slightly different nitrogen mole fractions of the compound semiconductor layer containing nitrogen and a V group element other than nitrogen as an absorption periodic modulation structure or a gain periodic modulation structure.
Thus, the invention described herein makes possible the advantages of (1) providing a method for fabricating a semiconductor optical integrated device in which a light emitting element, a light receiving element, and an optical waveguide element are easily integrated without the necessity of high-temperature heat treatment, (2) providing a semiconductor optical integrated device fabricated by such a method, (3) providing a method for fabricating a semiconductor optical integrated device in which a) a high-temperature long-time heat treatment process is not required, b) a light emitting element, a light receiving element, and an optical waveguide element can be simultaneously formed by one crystal growth step without the necessity of complicated processing, and c) a light emitting layer of the light emitting element, a light absorption layer of the light receiving element, and a core layer of the optical waveguide element continue as one layer without any displacement or seam between the layers thus reducing excessive light loss, and (4) providing a semiconductor optical integrated device in which a light emitting element, a light receiving element, and an optical waveguide element are formed together simultaneously.
The present invention is also applicable to methods for fabricating an end face window structure type semiconductor laser, a gain-coupled distributed feedback semiconductor laser provided with an absorptive diffraction grating, and a gain-coupled distributed feedback semiconductor laser provided with a gain diffraction grating.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.